Google is the customer here. They happen to be a very big customer who is aggregating and organizing adspace inventory for thousands of their customers. But as with any such relationship, they are free to walk away at any time.

And yes it sucks; but there must be advertisers out there who think that a site that gets 10,000 impressions a day and is linked from most of the popular news sites is worth something; selling to them is a better move than whining about how Google doesn't love you anymore.